These experiments are designed to define in detail the structure of small intestinal epithelial tight junctions in normal and perturbed model states and to relate variations in tight junction structure and permeability. To accomplish these goals, electron microscopic, freeze fracture, ionic and macromolecular tracer, and electrophysiological experiments will be conducted in parallel. First linear density of tight junctions along the crypt villus axis will be quantitated and, along with other data concerning junctional structure, will be utilized to prepare a hypothesis describing the partitioning of passive ion permeability along this axis. We will also study structural events and their functional correlates with the loss of cells from the epithelial sheet-both in normal tissues and in vitro model systems. Thirdly, to increase our ability to evaluate functional parameters of tight junction structure we will attempt to define new, small candidate tracer molecules. Forthly, we will study in detail the mechanisms by which absorptive cells under osmotic loads manipulate their functional and structural characteristics - this model will be examined during both short-term and long-term phases of this response. We will also examine the functional significance of abberant lateral tight junction strands both in a region where they normally occur and in a model system in which they are induced. In addition we will study functional and structural correlates of epithelial restitution after limited well defined damage. Also goblet cell mucous release will be synchronized with cholera toxin to investigate the structural and functional effects that the state of mucous release has on goblet cell tight junctions. Lastly we will utilize new techniques to help us gain insight, on a molecular level, into physical events wich occur as absorptive cell plasma membranes are fractured and how these events may relate to the images which we produce with this technique. These experiments, designed to correlate tight junction structure in normal and abnormal states with mucosal permeability, may yield insights into small intestinal epithelial function.